It is known in the art relating to internal combustion engines that by operating an engine with a less than stoichiometric (lean) mixture of fuel and air, efficiency of the engine is improved. This means that for a given amount of work performed by the engine, less fuel will be consumed, resulting in improved fuel efficiency. It is also well known that reduction of NOx emissions when the fuel rate is lean has been difficult to achieve, resulting in an almost universal use of stoichiometric operation for exhaust control of automotive engines. By operating an engine with a stoichiometric mixture of fuel and air, fuel efficiency is good and NOx emission levels are reduced by over 90% once the vehicle catalyst reaches operating temperatures.
Recent developments in catalysts and engine control technologies have allowed lean operation of the engine, resulting in improved fuel efficiency and acceptable levels of NOx emissions. One such development is a NOx adsorber (also termed a “lean NOx trap” or “LNT”), which stores NOx emissions during fuel lean operations and allows release of the stored NOx during fuel rich conditions with conventional three-way catalysis to nitrogen and water. The adsorber has limited storage capacity and must be regenerated with a fuel rich reducing “pulse” as it nears capacity. It is desirable to control the efficiency of the regeneration event of the adsorber to provide optimum emission control and minimum fuel consumption. Various strategies have been proposed.
Techniques are known for adsorbing NOx (trapping) when the air-fuel ratio of the exhaust gas flowing into the NOx adsorbent is lean and releasing the adsorbed NOx (regenerating) when the air-fuel ratio of the exhaust gas flowing into the NOx adsorbent becomes rich wherein the amount of NOx adsorbed in the NOx adsorbent may be estimated from the engine load and the engine rotational speed. When the amount of the estimated NOx becomes the maximum NOx adsorption capacity of the NOx adsorbent, the air-fuel ratio of the exhaust gas flowing into the NOx adsorbent is made rich. Determination of a regeneration phase may also be on the basis of individual operating cycles of the internal combustion engine.
It is also known to estimate how full the LNT is by estimating the amount of NOx flowing into the LNT using a pre-LNT oxygen sensor. It is also known to schedule LNT regeneration based on estimations of accumulated NOx mass and engine load and speed operating condition probabilities.
Commonly assigned U.S. Pat. No. 6,293,092 to Ament et al. entitled “NOx adsorber system regeneration fuel control” discloses a method for controlling regeneration fuel supplied to an internal combustion engine operating with a lean fuel-air mixture during sequential rich mixture regeneration events of a NOx adsorber in which NOx emissions collected by the adsorber are purged to provide optimum emissions control and minimum fuel consumption. The method monitors the exhaust gases flowing out of the adsorber during the regeneration event to detect when the fuel-air mixture to the engine is within an excessively lean or rich range. When the sensed exhaust gases contain an excessively lean fuel-air mixture, fuel is increased to the engine. Fuel is decreased when the sensed exhaust gases contain an excessively rich fuel-air mixture. The fuel can be increased or decreased by adjusting the duration or fuel rate of the regeneration event. U.S. Pat. No. 6,293,092 is hereby incorporated by reference.
In the art related to spark ignition direct injection (SIDI) engines, it is known to operate the engine in a stratified charge mode (very lean operation) in a lower range of engine output and in a homogeneous mode (less lean, stoichiometric, or rich of stoichiometric operation) in a higher range of engine power output with an intermediate zone wherein the cylinders operate in a combination of stratified charge and homogeneous charge combustion. Such engine operation may generally be referred to as mixed mode operation. In the stratified charge mode, the fuel is injected during the piston compression stroke, preferably into a piston bowl from which it is directed to a spark plug for ignition near the end of the compression stroke. The combustion chambers contain stratified layers of different air/fuel mixtures. The stratified mode generally includes strata containing a stoichiometric or rich air/fuel mixture nearer the spark plug with lower strata containing progressively leaner air/fuel mixtures. In the homogeneous charge mode, fuel is injected directly into each cylinder during its intake stroke and is allowed to mix with the air charge entering the cylinder to form a homogeneous charge, which is conventionally ignited near the end of the compression stroke. The homogenous mode generally includes an air/fuel mixture that is stoichiometric, lean of stoichiometric or rich of stoichiometric.
Commonly assigned U.S. Pat. No. 7,181,908, the disclosure of which is hereby incorporated by reference herein in its entirety, describes a method to control a direct-injection gasoline engine during LNT regeneration events thereby improving driveability by adapting fueling to account for pumping losses resulting from higher throttling at homogeneous operation. Further, commonly assigned U.S. Pat. No. 7,181,902, the disclosure of which is hereby incorporated by reference herein in its entirety, describes a method to control a direct-injection gasoline engine during LNT regeneration events thereby improving driveability by timing transitions to homogeneous operation in accordance with fuel/air equivalence ratio considerations.
There remains a need in the art for a LNT regeneration control strategy, particularly for mixed mode spark ignition direct injection (SIDI) engines, that enables LNT regeneration without adversely impacting driveability or NOx emissions at the tailpipe.